As methods for preventing corrosion with high durability in a building structure and in a civil engineering structure, there are hot-dip galvanization and a coating method with an inorganic zinc coating material or an organic zinc coating material, each of which prevents corrosion by causing a sacrificial anode reaction through a use of a potential difference from an iron substrate, a zinc containing metal spraying method and the like.
The hot-dip galvanization is used for an exterior part in particular, a roof and the like of a building structure and it is used in wide applications such as an upper structure (box girder) of a steel bridge, a bearing of a steel bridge, a support of a guardrail, a support of a road sign and the like in a civil engineering structure.
However, the hot-dip galvanization, which is although widely used for corrosion prevention, has the following drawback. Precisely, since a product to the hot-dip galvanization is normally immersed in a galvanizing bath in a factory, the product is limited in its size depending on the bath size, and thus, a large product cannot be treated. Further, since the galvanizing bath for the hot-dip galvanization has the temperature of 430° C. to 510° C., the product (steel material) suffers thermal deformation, which sometimes necessitates a correction work for ensuring flatness of the galvanized product. In addition, the correction work needs a careful attention to a crack and a damage of a galvanized surface.
Further, the hot-dip galvanization often uses distilled zinc as a commonly used zinc metal, but regarding the distilled zinc, it has been pointed out that cadmium and lead contained therein may be mixed in galvanization. Accordingly, the distilled zinc tends to be used so carefully according to the RoHS directive (Restricting the use Of Hazardous Substances) that has recently been widespread in European countries.
The coating methods with the inorganic zinc coating material and with the organic zinc coating material are also widely used as the sacrificial corrosion prevention method. Careful control should be paid for inorganic coating material to be prepared by such as blasting so as to ensure its adhesiveness of a product (steel material), because the inorganic coating material is lack of adhesiveness. In particular, corrosion proof performance were expected enough to equivalent to that of zinc plating, it would mandatory require to make a coating film should be thick, resulting in requiring to maintain adhesiveness for long term, however, it is really tough to be achieved. On the other hand, the coating using the organic zinc coating material where organic resin is used as a binder makes it possible to favorably ensure adhesiveness to a product (steel material) to be coated therewith, compared with the coating using the inorganic zinc coating material.
In addition to the above-described coatings with zinc-containing coating materials, there is a technique, in which zinc, aluminum, magnesium or the like exhibiting a sacrificial anode reaction on iron is sprayed so that a thermal spray metallic coating made of this material is formed on a metal body, as shown in Patent Literature 1, for instance. The thermal sprayed metal in this case only needs to exhibit the sacrificial anode reaction on iron as the metal body. Accordingly, favorable corrosion prevention can be achieved with any metal selected from a pure metal, an alloy or a pseudo alloy.
In the art to prevent corrosion by thermally spraying the metal material exhibiting the sacrificial anode reaction, melted metal is just sprayed with a thermal spraying gun at high temperature so as to adhere on a product surface, which has been treated by blasting or by a power tool in advance. Therefore, unlike the aforesaid hot-dip galvanization, since the temperature of the product does not become high, thermal deformation of the metal body can be favorably prevented. In addition to this, the product is not limited in size, because there is not a plating bath, which would cause such limitation.
However, even in the thermal spraying, molten metal such as aluminum or magnesium, which is thermally sprayed with the thermal spraying gun, needs to be sprayed preferably, at a right angle to a sprayed surface. This makes it difficult to do this work at a narrow space where the thermal spraying gun cannot be used or at a worksite where cylinders of gases or power sources necessary for the thermal spraying cannot be brought.
Therefore, as a versatile method for sacrificial corrosion prevention of iron, a method with zinc as a sacrificial anticorrosive component has been widely executed. However, the use of zinc has a problem that it requires refining and the like because a raw material ore contains lead and cadmium. Further, zinc can be said to be a preferable material in view of its capability of quickly exhibiting a sacrificial anode reaction on an iron substrate, but on the other hand, because of its quick elution, its durability does not last so long, which has given rise to a problem that frequent maintenance such as repairing is required in order to prevent corrosion by maintaining the sacrificial anode reaction stably for a long period. Therefore, there is a necessity for a technique with a metal, which is capable to exhibit a sacrificial anticorrosive reaction that is sufficient to the same degree as zinc does and which is able to keep durability of a product.
Patent Literature 2 discloses a technique, in which particulates of metal or an alloy in the form of aerosol having sacrificial corrosion protection is sprayed to a steel material surface to form a coating film. Further Patent Literature 2 describes that the metal or the alloy having the sacrificial corrosion protection preferably contains one kind or two or more kinds selected from zinc, aluminum and magnesium.
However, the technique disclosed by Patent Literature 2 is to form the coating film by making the particulates of the metal or the alloy in the aerosol form collide with the steel material surface by spraying the particulates to a surface for corrosion prevention. So, this technique requires an aerosol spraying facility (including an aerosol chamber, a classification chamber, an injection nozzle, a gas generating chamber, a film forming chamber, a vacuum pump) as well as inert gas such as helium or nitrogen. Therefore, on-site formation of the coating film cannot be easily executed also in this technique.
Incidentally, Patent Literature 3 discloses a composition for powder coating (powder baking coating), in which a to 900 parts by weight zinc powder is compounded with 100 parts by weight epoxy resin. This is just to provide a powder baking coating material and also requires a specific facility. That is to say, such coating cannot be easily executed.